Systems and method for controlling auto-ignition

ABSTRACT

Various methods and systems are provided for maintaining combustion stability in a multi-fuel engine. In one example, a system comprises a first fuel system to deliver liquid fuel to at least one cylinder of an engine, a second fuel system to deliver gaseous fuel to the at least one cylinder, and a controller. The controller is configured to supply the gaseous fuel to the at least one cylinder, inject the liquid fuel to the at least one cylinder thereby to ignite the liquid fuel and the gaseous fuel in the at least one cylinder via compression-ignition, and adjust an amount of the gaseous fuel relative to an amount of the liquid fuel based on a measured parameter associated with auto-ignition of end gases subsequent to the compression-ignition of the liquid fuel.

FIELD

Embodiments of the subject matter disclosed herein relate to an engine,engine components, and an engine system, for example.

BACKGROUND

Engines may be configured to operate with more than one fuel type. Forexample, engines may operate with liquid fuel, such as diesel, andgaseous fuel, such as natural gas. A mixture of gaseous fuel and airprovided to a cylinder of an engine may be ignited when liquid fuel isinjected into the cylinder. The compression of the cylinder ignites theliquid fuel and the gas/air mixture burns from the initiation sitecreated by the liquid fuel combustion, forming a flame front thatcompresses and heats the unburned mixture ahead of the flame front.Under certain conditions where the temperature and pressure of theunburned gases reach an auto-ignition limit, combustion may be initiatedbefore the main flame front can initiate combustion, resulting in asecondary flame front. If the secondary flame front and main flame frontmeet, cylinder vibrations may occur that can lead to engine degradation.Further, uncontrolled auto-ignition may result in high in-cylinderpressure, which may lead to engine degradation and potentially higheremissions due to higher in-cylinder temperatures.

BRIEF DESCRIPTION

In one embodiment, a system comprises a first fuel system to deliverliquid fuel to at least one cylinder of an engine, a second fuel systemto deliver gaseous fuel to the at least one cylinder, and a controller.The controller is configured to control the supply of gaseous fuel tothe at least one cylinder, control the injection of liquid fuel to theat least one cylinder to ignite the liquid fuel (and thereby to ignitethe gaseous fuel) in the at least one cylinder via compression-ignition,and adjust an amount of the gaseous fuel relative to an amount of theliquid fuel based on a measured parameter associated with auto-ignitionof end gases subsequent to the compression-ignition of the liquid fuel.(End gases are the part of the fuel-air mixture that has been introducedinto the cylinder but not yet consumed in the flame-front reaction ofthe compression-ignition of the liquid fuel.)

In this way, upon indication that auto-ignition of end gases subsequentto the compression-ignition of the liquid fuel is occurring, the amountof gaseous fuel relative to the amount of liquid fuel may be decreasedto reduce the auto-ignition. In some examples, the injection timing ofthe liquid fuel injection may be adjusted (e.g., retarded) prior toadjusting the amount of gaseous fuel relative to the amount of liquidfuel following the indication of auto-ignition.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows a schematic diagram of a rail vehicle with an engineaccording to an embodiment of the invention.

FIG. 2 shows a schematic diagram of a single cylinder of the engine ofFIG. 1.

FIG. 3 shows a schematic diagram of the engine of FIGS. 1 and 2.

FIGS. 4-6 are flow charts illustrating methods for controlling theengine of FIGS. 1-3.

DETAILED DESCRIPTION

Auto-ignition of the end gases following ignition of the injected fuelmay be a function of many different parameters, such as ignition (e.g.,injection) timing, air temperature in the cylinder, and in-cylindercombustion mixture. Thus, to reduce auto-ignition, multiple factors maybe adjusted, such as ignition timing. With respect to dual fuel engineswhich use diesel injection to ignite a premixed air/natural gas mixture,the substitution ratio is another factor which may affect auto-ignition.As the substitution ratio increases (where more energy is derived frompremixed natural gas compared to the non-premixed diesel), thelikelihood of auto-ignition combustion increases. According toembodiments disclosed herein, the substitution ratio is used as anotherlever to adjust to control auto-ignition. If auto-ignition is detectedand other methods such as retarding diesel injection timing do notreduce the auto-ignition to acceptable levels, the substitution ratiomay be decreased (e.g., gaseous fuel amount reduced and/or liquid fuelamount increased) until auto-ignition is eliminated. FIGS. 1-3illustrate an engine configured to operate with liquid and gaseous fuel.The engine of FIGS. 1-3 may be controlled by a controller according tomethods and routines illustrated in FIGS. 4-6.

The approach described herein may be employed in a variety of enginetypes, and a variety of engine-driven systems. Some of these systems maybe stationary, while others may be on semi-mobile or mobile platforms.Semi-mobile platforms may be relocated between operational periods, suchas mounted on flatbed trailers. Mobile platforms include self-propelledvehicles. Such vehicles can include on-road transportation vehicles, aswell as mining equipment, marine vessels, rail vehicles, and otheroff-highway vehicles (OHV). For clarity of illustration, a locomotive isprovided as an example of a mobile platform supporting a systemincorporating an embodiment of the invention.

Before further discussion of the approach for reducing auto-ignition ofend gases in a dual fuel engine, an example of a platform is disclosedin which an engine may be configured for a vehicle, such as a railvehicle. For example, FIG. 1 shows a block diagram of an exampleembodiment of a vehicle system 100, herein depicted as a rail vehicle106 (e.g., locomotive), configured to run on a rail 102 via a pluralityof wheels 112. As depicted, the rail vehicle 106 includes an engine 104.In other non-limiting embodiments, the engine 104 may be a stationaryengine, such as in a power-plant application, or an engine in a marinevessel or other off-highway vehicle propulsion system as noted above.

The engine 104 receives intake air for combustion from an intake passage114. The intake passage 114 receives ambient air from an air filter 160that filters air from outside of the rail vehicle 106. Exhaust gasresulting from combustion in the engine 104 is supplied to an exhaustpassage 116. Exhaust gas flows through the exhaust passage 116, and outof an exhaust stack of the rail vehicle 106. In one example, the engine104 is a diesel engine that combusts air and diesel fuel throughcompression ignition. In another example, engine 104 is a dual ormulti-fuel engine that may combust a mixture of gaseous fuel and airupon injection of diesel fuel during compression of the air-gaseous fuelmix. In other non-limiting embodiments, the engine 104 may additionallycombust fuel including gasoline, kerosene, natural gas, biodiesel, orother petroleum distillates of similar density through compressionignition (and/or spark ignition).

In one embodiment, the rail vehicle 106 is a diesel-electric vehicle. Asdepicted in FIG. 1, the engine 104 is coupled to an electric powergeneration system, which includes an alternator/generator 122 andelectric traction motors 124. For example, the engine 104 is a dieseland/or natural gas engine that generates a torque output that istransmitted to the alternator/generator 122 which is mechanicallycoupled to the engine 104. In one embodiment herein, engine 104 is amulti-fuel engine operating with diesel fuel and natural gas, but inother examples engine 104 may use various combinations of fuels otherthan diesel and natural gas.

The alternator/generator 122 produces electrical power that may bestored and applied for subsequent propagation to a variety of downstreamelectrical components. As an example, the alternator/generator 122 maybe electrically coupled to a plurality of traction motors 124 and thealternator/generator 122 may provide electrical power to the pluralityof traction motors 124. As depicted, the plurality of traction motors124 are each connected to one of a plurality of wheels 112 to providetractive power to propel the rail vehicle 106. One example configurationincludes one traction motor per wheel set. As depicted herein, six pairsof traction motors correspond to each of six pairs of motive wheels ofthe rail vehicle. In another example, alternator/generator 122 may becoupled to one or more resistive grids 126. The resistive grids 126 maybe configured to dissipate excess engine torque via heat produced by thegrids from electricity generated by alternator/generator 122.

In some embodiments, the vehicle system 100 may include a turbocharger120 that is arranged between the intake passage 114 and the exhaustpassage 116. The turbocharger 120 increases air charge of ambient airdrawn into the intake passage 114 in order to provide greater chargedensity during combustion to increase power output and/orengine-operating efficiency. The turbocharger 120 may include acompressor (not shown) which is at least partially driven by a turbine(not shown). While in this case a single turbocharger is included, thesystem may include multiple turbine and/or compressor stages.Additionally or alternatively, in some embodiments, a supercharger maybe present to compress the intake air via a compressor driven by a motoror the engine, for example. Further, in some embodiments, a charge aircooler may be present between the compressor of the turbocharger orsupercharger and intake manifold of the engine. The charge air coolermay cool the compressed air to further increase the density of thecharge air.

In some embodiments, the vehicle system 100 may further include anaftertreatment system coupled in the exhaust passage upstream and/ordownstream of the turbocharger 120. In one embodiment, theaftertreatment system may include a diesel oxidation catalyst (DOC) anda diesel particulate filter (DPF). In other embodiments, theaftertreatment system may additionally or alternatively include one ormore emission control devices. Such emission control devices may includea selective catalytic reduction (SCR) catalyst, three-way catalyst,NO_(x) trap, or various other devices or systems.

The vehicle system 100 may further include an exhaust gas recirculation(EGR) system 130 coupled to the engine 104, which routes exhaust gasfrom an exhaust passage 116 of the engine 104 to the intake passage 114downstream of the turbocharger 120. In some embodiments, the exhaust gasrecirculation system 130 may be coupled exclusively to a group of one ormore donor cylinders of the engine (also referred to a donor cylindersystem). As depicted in FIG. 1, the EGR system 130 includes an EGRpassage 132 and an EGR cooler 134 to reduce the temperature of theexhaust gas before it enters the intake passage 114. By introducingexhaust gas to the engine 104, the amount of available oxygen forcombustion is decreased, thereby reducing the combustion flametemperatures and reducing the formation of nitrogen oxides (e.g.,NO_(x)).

In some embodiments, the EGR system 130 may further include an EGR valvefor controlling an amount of exhaust gas that is recirculated from theexhaust passage 116 of the engine 104 to the intake passage 114 ofengine 104. The EGR valve may be an on/off valve controlled by thecontroller 110, or it may control a variable amount of EGR, for example.As shown in the non-limiting example embodiment of FIG. 1, the EGRsystem 130 is a high-pressure EGR system. In other embodiments, thevehicle system 100 may additionally or alternatively include alow-pressure EGR system, routing EGR from downstream of the turbine toupstream of the compressor.

As depicted in FIG. 1, the vehicle system 100 further includes a coolingsystem 150. The cooling system 150 circulates coolant through the engine104 to absorb waste engine heat and distribute the heated coolant to aheat exchanger, such as a radiator 152. A fan 154 may be coupled to theradiator 152 in order to maintain an airflow through the radiator 152when the vehicle 106 is moving slowly or stopped while the engine isrunning. In some examples, fan speed may be controlled by a controller,such as controller 110. Coolant which is cooled by the radiator 152enters a tank 156. The coolant may then be pumped by a water, orcoolant, pump (not shown) back to the engine 104 or to another componentof the vehicle system, such as the EGR cooler and/or charge air cooler.

The rail vehicle 106 further includes an engine controller 110 (referredto hereafter as the controller) to control various components related tothe rail vehicle 106. As an example, various components of the vehiclesystem may be coupled to the controller 110 via a communication channelor data bus. In one example, the controller 110 includes a computercontrol system. The controller 110 may additionally or alternativelyinclude a memory holding non-transitory computer readable storage media(not shown) including code for enabling on-board monitoring and controlof rail vehicle operation. In some examples, controller 110 may includemore than one controller each in communication with one another, such asa first controller to control the engine and a second controller tocontrol other operating parameters of the locomotive (such as tractivemotor load, blower speed, etc.). The first controller may be configuredto control various actuators based on output received from the secondcontroller and/or the second controller may be configured to controlvarious actuators based on output received from the first controller.

The controller 110 may receive information from a plurality of sensorsand may send control signals to a plurality of actuators. The controller110, while overseeing control and management of the engine 104 and/orrail vehicle 106, may be configured to receive signals from a variety ofengine sensors, as further elaborated herein, in order to determineoperating parameters and operating conditions, and correspondinglyadjust various engine actuators to control operation of the engine 104and/or rail vehicle 106. For example, the engine controller 110 mayreceive signals from various engine sensors including, but not limitedto, engine speed, engine load, intake manifold air pressure, boostpressure, exhaust pressure, ambient pressure, ambient temperature,exhaust temperature, particulate filter temperature, particulate filterback pressure, engine coolant pressure, gas temperature in the EGRcooler, or the like. Correspondingly, the controller 110 may controlengine 104 and/or the rail vehicle 106 by sending commands to variouscomponents such as the traction motors 124, the alternator/generator122, fuel injectors, or the like. For example, the controller 110 maycontrol the timing and/or duration of liquid fuel injection, and/or thetiming and/or duration of gaseous fuel supply, as described below. Otheractuators may be coupled to various locations in the rail vehicle.

FIG. 2 depicts an embodiment of a combustion chamber, or cylinder 200,of a multi-cylinder internal combustion engine, such as the engine 104described above with reference to FIG. 1. Cylinder 200 may be defined bya cylinder head 201, housing the intake and exhaust valves and liquidfuel injector, described below, and a cylinder block 203.

The engine may be controlled at least partially by a control systemincluding controller 110 which may be in further communication with avehicle system, such as the vehicle system 100 described above withreference to FIG. 1. As described above, the controller 110 may furtherreceive signals from various engine sensors including, but not limitedto, engine speed, engine load, boost pressure, exhaust pressure, ambientpressure, CO₂ levels, exhaust temperature, NO_(x) emission, enginecoolant temperature (ECT) from temperature sensor 230 coupled to coolingsleeve 228, etc. Correspondingly, the controller 110 may control thevehicle system by sending commands to various components such asalternator, cylinder valves, throttle, fuel injectors, etc.

The cylinder (i.e., combustion chamber) 200 may include cylinder liner204 with a piston 206 positioned therein. The piston 206 may be coupledto a crankshaft 208 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. In someembodiments, the engine may be a four-stroke engine in which each of thecylinders fires in a firing order during two revolutions of thecrankshaft 208. In other embodiments, the engine may be a two-strokeengine in which each of the cylinders fires in a firing order during onerevolution of the crankshaft 208.

The cylinder 200 receives intake air for combustion from an intakeincluding an intake passage 210. The intake passage 210 receives intakeair via an intake manifold. The intake passage 210 may communicate withother cylinders of the engine in addition to the cylinder 200, forexample, or the intake passage 210 may communicate exclusively with thecylinder 200.

Exhaust gas resulting from combustion in the engine is supplied to anexhaust including an exhaust passage 212. Exhaust gas flows through theexhaust passage 212, to a turbocharger in some embodiments (not shown inFIG. 2) and to atmosphere, via an exhaust manifold. The exhaust passage212 may further receive exhaust gases from other cylinders of the enginein addition to the cylinder 200, for example.

Each cylinder of the engine may include one or more intake valves andone or more exhaust valves. For example, the cylinder 200 is shownincluding at least one intake poppet valve 214 and at least one exhaustpoppet valve 216 located in an upper region of cylinder 200. In someembodiments, each cylinder of the engine, including cylinder 200, mayinclude at least two intake poppet valves and at least two exhaustpoppet valves located at the cylinder head.

The intake valve 214 may be controlled by the controller 110 via anactuator 218. Similarly, the exhaust valve 216 may be controlled by thecontroller 110 via an actuator 220. During some conditions, thecontroller 110 may vary the signals provided to the actuators 218 and220 to control the opening and closing of the respective intake andexhaust valves. The position of the intake valve 214 and the exhaustvalve 216 may be determined by respective valve position sensors 222 and224, respectively, and/or by cam position sensors. The valve actuatorsmay be of the electric valve actuation type or cam actuation type, or acombination thereof, for example.

The intake and exhaust valve timing may be controlled concurrently orany of a possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. In other embodiments, the intake and exhaust valves may becontrolled by a common valve actuator or actuation system, or a variablevalve timing actuator or actuation system. Further, the intake andexhaust valves may by controlled to have variable lift by the controllerbased on operating conditions.

In still further embodiments, a mechanical cam lobe may be used to openand close the intake and exhaust valves. Additionally, while afour-stroke engine is described above, in some embodiments a two-strokeengine may be used, where the intake valves are dispensed with and portsin the cylinder wall are present to allow intake air to enter thecylinder as the piston moves to open the ports. This can also extend tothe exhaust, although in some examples exhaust valves may be used.

In some embodiments, each cylinder of the engine may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, FIG. 2 shows the cylinder 200 is including a fuel injector 226.The fuel injector 226 is shown coupled directly to the cylinder 200 forinjecting fuel directly therein. In this manner, fuel injector 226provides what is known as direct injection of a fuel into combustioncylinder 200. The fuel may be delivered to the fuel injector 226 from afirst, liquid fuel system 232, including a fuel tank, fuel pumps, and afuel rail (described in more detail with respect to FIG. 3). In oneexample, the fuel is diesel fuel that is combusted in the engine throughcompression ignition. In other non-limiting embodiments, the fuel may begasoline, kerosene, biodiesel, or other petroleum distillates of similardensity through compression ignition (and/or spark ignition).

Further, each cylinder of the engine may be configured to receivegaseous fuel (e.g., natural gas) alternative to or in addition to dieselfuel. The gaseous fuel may be provided to cylinder 200 via the intakemanifold, as explained below. As shown in FIG. 2, the intake passage 210may receive a supply of gaseous fuel from a second, gaseous fuel system234, via one or more gaseous fuel lines, pumps, pressure regulators,etc., located upstream of the cylinder. In some embodiments, gaseousfuel system 234 may be located remotely from engine 104, such as on adifferent rail car (e.g., on a fuel tender car), and the gaseous fuelmay be supplied to the engine 104 via one or more fuel lines thattraverse the separate cars. However, in other embodiments gaseous fuelsystem 234 may be located on the same rail car as engine 104.

A plurality of gas admission valves, such as gas admission valve 236,may be configured to supply gaseous fuel from gaseous fuel system 234 toeach respective cylinder via respective intake passages. For example, adegree and/or duration of opening of gas admission valve 236 may beadjusted to regulate an amount of gaseous fuel provided to the cylinder.As such, each respective cylinder may be provided with gaseous fuel froman individual gas admission valve, allowing for individual cylindercontrol in the amount of gaseous fuel provided to the cylinders. Howeverposit some embodiments, a single-point fumigation system may be used,where gaseous fuel is mixed with intake air at a single point upstreamof the cylinders. In such a configuration, each cylinder may be providedwith substantially similar amounts of gaseous fuel. To regulate theamount of gaseous fuel provided by the single-point fumigation system,in some examples a gaseous fuel control valve may be positioned at ajunction between a gaseous fuel supply line and the engine intake airsupply line or intake manifold. The gaseous fuel control valve degreeand/or duration of opening may be adjusted to regulate the amount ofgaseous fuel admitted to the cylinders. In other examples, the amount ofgaseous fuel admitted to the cylinders in the single-point fumigationsystem may be regulated by another mechanism, such as control of agaseous fuel regulator, via control of a gaseous fuel pump, etc.

FIG. 3 illustrates multiple cylinders of engine 104, including cylinder200, cylinder 302, cylinder 304, and cylinder 306. While four cylindersarranged in-line are illustrated in FIG. 3, such an arrangement isnon-limiting, and other engine configurations are possible. For example,engine 104 may be a V-6, V-8, V-12, V-16, I-6, I-8, or other enginetype. Engine 104 may be supplied one or more of liquid fuel from liquidfuel system 232 and gaseous fuel from gaseous fuel system 234. As such,each cylinder of engine 104 includes a liquid fuel injector, includinginjector 226 as well as injectors 308, 310, and 312. Each liquid fuelinjector is supplied with liquid fuel from a common fuel rail 314.Common fuel rail 314 may be supplied with fuel from liquid fuel tank 320via supply line 316. The fuel may be provided at a high pressure via oneor more fuel pumps, such as pump 318. The liquid fuel in liquid fuelsystem 232 may be diesel fuel or another liquid fuel, such as gasoline,alcohol, etc. Further, while a common fuel rail system is illustrated inFIG. 3, a non-common rail unit pump injection system may be used.

Each cylinder of engine 104 may similarly include a gas admission valveto supply gaseous fuel, including gas admission valve 236 as well as gasadmission valves 322, 324, and 326. Each gas admission valve may bepositioned in an intake passage of a respective cylinder, or othersuitable location. The gas admission valves may be supplied gaseousfuel, such as natural gas, from a gaseous fuel passage 328. The gaseousfuel passage 328 may receive gaseous fuel from a gaseous fuel tank 330via a supply line 332. As explained previously, gaseous fuel tank 330may be located remotely from engine 104. However, in some embodiments,the individual gas admission valves may be dispensed with, and all thecylinders may be supplied with the same gaseous fuel/intake air mixturefrom an upstream single-point fumigation system.

Each liquid fuel injector of each cylinder, as well as each gasadmission valve of each cylinder, may be individually controlled by acontroller (such as controller 110) to enable individual cylindercontrol of the fuel supply. Accordingly, each cylinder may be operatedwith varying levels of liquid fuel and/or gaseous fuel. In someembodiments, the liquid fuel injectors may be controlled by a differentcontroller than the controller that controls the gas admission valves.Further, in a gaseous fumigation system, rather than controlling theindividual gas admission valves, a single gaseous fuel control valve orother gaseous fuel control element may be controlled by the controllerto regulate the amount of gaseous fuel admitted to the cylinders.

In an example, a mixture of gaseous fuel and air may be provided tocylinder 200 via intake passage 210 and, in some embodiments, gasadmission valve 236. Then, during compression, diesel fuel may beinjected to cylinder 200 via fuel injector 226. The diesel fuel may beignited via compression ignition and subsequently ignite the gaseousfuel.

During a standard combustion event in a multi-fuel engine, thecompression ignition of the liquid (e.g., diesel) fuel initiatescombustion of a mixture of air and fuel (such as gaseous fuel), causinga combustion flame front to develop that spreads across the cylinderconsuming unburned fuel/air mix. The flame front typically moves awayfrom the site of ignition and across the combustion chamber. However,under certain conditions a second flame front may develop away from thefirst flame front. The second flame front may occur after the liquidfuel combustion initiates combustion of the gaseous fuel/air mixture andresults from compressing and heating of the unburned gaseous fuel/airmixture ahead of the flame front to a high enough level that the end gasmixture auto-ignites ahead of the flame front. If the first flame frontmeets the second flame front, cylinder vibrations may occur that canresult in engine damage. Further, the auto-ignition of end gases maydegrade emissions.

To detect the presence of auto-ignition in one or more cylinders ofengine 104, one or more vibration and/or acceleration sensors 334 may bepresent. Auto-ignition sensor 334 may be configured to detect vibrationsto the engine caused by auto-ignition. As such, auto-ignition sensor 334may include an accelerometer or other motion sensor, a microphone orother acoustic sensor, a laser, and/or other sensor. To detectauto-ignition on an individual cylinder level, each cylinder may includean auto-ignition sensor. In other examples, one auto-ignition sensor maybe used, and the cylinder(s) undergoing auto-ignition may be identifiedbased on correlation to cylinder firing order/engine position.

Engines configured to operate with both liquid and gaseous fuel may beoperated with as much gaseous fuel as possible while still maintainingrequested engine power. For example, in standard liquid-fueled engines,such as diesel engines, 100% of produced engine power may be derivedfrom combustion of diesel fuel. In multi-fuel engines, a portion of theengine power may be derived from gaseous fuel while the remaining enginepower may be derived from liquid fuel. For example, as much as 80% ofproduced engine power may be derived from combustion of gaseous fuel,with the remaining 20% of power derived from the combustion of dieselfuel. The amount of gaseous fuel “substituted” for the liquid fuel maybe referred to as the substitution ratio. The substitution ratio mayreflect the portion of engine power derived from gaseous fuel. Forexample, a substitution ratio of 80 indicates 80% of the power isderived from gaseous fuel, while a substitution ratio of 50 indicates50% of the power is derived from gaseous fuel. A substitution ratio of 0indicates liquid-only operation.

However, as the substitution ratio increases (e.g., as the amount ofgaseous fuel present during combustion increases and/or the amount ofliquid fuel present decreases), the auto-ignition of the end gases mayincrease. According to embodiments described herein, auto-ignition ofend gasses may be monitored (based on output from the auto-ignitionsensor, for example) and if auto-ignition is detected, the substitutionratio may be decreased until the auto-ignition ceases. Once theauto-ignition has stopped, the substitution ratio may be increased againuntil a maximum desired substitution ratio that does not produceauto-ignition is identified.

FIG. 4 is a flow chart illustrating a method 400 for controlling amulti-fuel engine configured to operate with one or more of liquid andgaseous fuel, such as engine 104 of FIGS. 1-3. Method 400 may be carriedout according to non-transitory instructions stored in the memory of acontrol system, such as controller 110. At 402, method 400 includesdetermining engine operating parameters. The determined engine operatingparameters may include engine speed, engine load, current fuelsubstitution ratio, relative fuel levels in each fuel tank, etc. At 404,method 400 includes setting a fuel substitution ratio based on theoperating parameters. The substitution ratio may be set based on enginetemperature, desired fuel type, notch throttle position, relative fuellevels in each fuel tank (e.g., if the level of gaseous fuel is below athreshold, more liquid fuel may be used), vehicle location (e.g.,whether the vehicle is in a tunnel), and/or other parameters. At 406,the gaseous and/or liquid fuel is supplied to each cylinder of theengine at the set substitution ratio. In some examples, the setsubstitution ratio may be the same for all cylinders. In other examples,one or more cylinders may have different substitution ratios.

If the substitution ratio is greater than zero (e.g., if at least somegaseous fuel is supplied), the gaseous fuel may be premixed with intakeair and combusted due to compression ignition of the injected liquidfuel. The liquid fuel may be supplied via stratified injection, wherethe liquid fuel is not homogeneous throughout the combustion chamber,but instead varies in a controlled way across the volume of thecylinder. For example, the liquid fuel may be injected into a particularvolume of the combustion chamber (e.g., piston crown). The liquid fuelmay be injected at a prescribed time during the combustion cycle (suchas the end of the compression stroke or beginning of the power stroke)such that the liquid fuel ignites quickly after injection due toincreased cylinder temperature at high compression levels. The ignitedliquid fuel may then ignite the premixed gaseous fuel and air.

At 408, output from one or more auto-ignition sensors is monitored todetermine if auto-ignition is present in any of the cylinders. Asexplained above, the auto-ignition sensor (such as auto-ignition sensor334) may detect engine vibrations resulting from auto-ignition ofcylinder end gases (e.g., ignition of gaseous fuel and air ahead of theflame front resulting from the ignition of the injected liquid fuel).The detected vibrations that indicate auto-ignition may be vibrationsabove a threshold level of normal engine vibrations, and may vary basedon engine speed or other parameters. At 410, method 400 determines ifauto-ignition of end gases is detected in at least one cylinder. Ifauto-ignition is detected, method 400 proceeds to 412 to initiate anauto-ignition reduction routine, which is explained below with respectto FIG. 5.

If auto-ignition is not detected, method 400 proceeds to 414 to continueto operate with the set substitution ratio and liquid fuel injectiontiming. At 416 method 400 optionally includes monitoring for gasadmission valve degradation, which is explained below with respect toFIG. 6.

FIG. 5 is a flow chart illustrating a routine 500 for reducingauto-ignition. Routine 500 may be executed in response to an indicationof auto-ignition, as explained above with respect to method 400. Routine500 includes, at 502, identifying the cylinder(s) undergoingauto-ignition. As explained above, cylinder identification may includedetermining which auto-ignition sensor is indicating auto-ignition isoccurring, if more than auto-ignition sensor is present, and/orcorrelating the timing of the detected auto-ignition to each cylinder'spredetermined firing time.

Then, for each cylinder where auto-ignition has been detected, routine500 includes identifying the level of auto-ignition at 504. The level ofauto-ignition may be based on the intensity, frequency, etc., of thevibrations detected by the auto-ignition sensor. At 506, routine 500determines if the auto-ignition level is above a threshold level. Thethreshold level may be a high level of auto-ignition that may causeengine degradation if allowed to continue, and/or may be a level ofauto-ignition that may be difficult to reduce in a timely manner. If theauto-ignition level is above the threshold, routine 500 proceeds to 508to operate with liquid fuel only, and shut down the supply of gaseousfuel. The supply of gaseous fuel may be shut down to all the cylinders,or only to the cylinder(s) undergoing auto-ignition. To maintain enginepower demand, the amount of liquid fuel supplied to the cylinders isincreased. At 510, operation at the set substitution ratio (e.g.,including a supply of gaseous fuel) may be resumed once auto-ignitionhas stopped for a predetermined amount of time. Routine 500 then ends.

Returning to 506, if it is determined that the auto-ignition is notabove the threshold level, routine 500 proceeds to 512 to retard liquidfuel injection timing. In some examples, the liquid fuel injectiontiming may be retarded by predetermined amount. In other examples, theliquid fuel injection timing may be incrementally retarded untilauto-ignition stops. In still further examples, the liquid fuelinjection timing may be maintained, and routine 500 may immediatelyproceed to 516, explained below.

After adjusting the liquid fuel injection timing, routine 500determines, at 514, if the auto-ignition has stopped. If yes, routine500 ends. If the retarding of the liquid fuel injection timing has notstopped the auto-ignition (and, in some examples, if the injectiontiming can be retarded no more), routine 500 proceeds to 516 toincrementally reduce the substitution ratio until the auto-ignitionstops. The substitution ratio may be reduced in a suitable manner. Forexample, the substitution ratio may be reduced by a predeterminedamount, such as 5%, and the engine may be monitored for auto-ignitionafter each incremental reduction. To reduce the substitution ratio, theamount of supplied gaseous fuel may be reduced, the amount of suppliedliquid fuel may be increased, or both. When the substitution ratio isdecreased, engine power is maintained via an increase in the amount ofliquid fuel supplied, for example. In some examples, the substitutionratio may be decreased to zero.

At 518, after auto-ignition has stopped, the substitution ratio may beincrementally increased back towards the set substitution ratio.However, to ensure auto-ignition does not occur again, the substitutionratio may be incremented by a smaller amount, such as 2%. Further, thesubstitution ratio may be increased to a ratio closer to but lower thanthe set substitution ratio. The substitution ratio may be increaseduntil a maximum desired substitution ratio is reached (such as the setsubstitution ratio), or until auto-ignition occurs again. Ifauto-ignition occurs again, as determined at 520, routine 500 loops backto 516 to again incrementally decrease the substitution ratio until theauto-ignition stops. The process of reducing the substitution ratiountil the auto-ignition stops and subsequently increasing thesubstitution ratio may be repeated until a maximum substitution ratiothat is as close to the set substitution ratio as possible but does notcause auto-ignition is reached.

If, at 520, auto-ignition is not detected again, routine 500 optionallyincludes advancing the liquid fuel injection timing back towards theoriginal (non-retarded) injection timing. Routine 500 then ends.

Thus, routine 500 mitigates auto-ignition of end gases followingcompression ignition of injected liquid fuel by first retarding theinjection timing of the liquid fuel. The liquid fuel injection timingmay be initially set to within a margin that prevents the auto-ignitionwithout causing misfire and meets designated emissions levels. Ifauto-ignition following compression ignition is detected, the injectiontiming may be retarded. If the retarded injection timing does not reducethe auto-ignition, the substitution ratio may be decreased (e.g., lessgaseous fuel and/or more liquid fuel may be supplied). After theauto-ignition has stopped, either the injection timing may be advancedagain, the substitution ratio may be increased again, or both, so thatthe injection timing and substitution ratio are as close to the optimal,predetermined settings as possible without causing the auto-ignitionfollowing compression ignition.

The control of liquid fuel injection timing and gaseous fuelsubstitution ratio may be made on a per-cylinder basis in someembodiments. Accordingly, if only one cylinder is undergoing significantauto-ignition following compression ignition, only the injection timingand/or substitution ratio of that cylinder may be adjusted, while theremaining cylinders are maintained at the optimal settings. However, insome embodiments, even if auto-ignition is not detected in each cylinder(e.g., if auto-ignition is detected in only a subset of the cylinders),each cylinder may undergo the same injection timing and/or substitutionratio adjustment to control the detected auto-ignition. Further, theintensity of the auto-ignition may be monitored. In some examples, ifthe auto-ignition is of relatively high intensity, it may be quicklyreduced by completely stopping the gaseous fuel supply (either to thecylinder undergoing the auto-ignition, or to all cylinders). Then, onceconditions resulting the auto-ignition have changed (e.g., enginetemperature has decreased), the gaseous fuel supply may be resumed.

Turning now to FIG. 6, a routine 600 for monitoring gas admission valvedegradation status is illustrated. Routine 600 may be executed duringgaseous fuel operation without auto-ignition, such as during the method400 described above, in an engine system that includes a plurality ofgas admission valves, each for admitting gaseous fuel to a respectivecylinder. At 602, routine 600 includes monitoring cylinder output at theset substitution ratio. The cylinder output may be a suitable outputthat indicates the power produced by the cylinder during combustion, andmay include individual cylinder exhaust temperature, overall enginepower (e.g., load placed on alternator), engine speed, or otherparameter. At 604, the produced output is compared to an expected outputfor the given substitution ratio. If the output is not different thanthe expected output, routine 600 proceeds to 606 to indicate that nodegradation to the gas admission valves is present, and routine 600ends.

If the produced cylinder output is different than expected, routine 600proceeds to 608 to determine if the produced output is more or less thanthe expected output. If the produced output is more than expected,routine 600 proceeds to 610 to decrease the amount of gaseous fuel, andin some embodiments liquid fuel, supplied to the cylinder until thecylinder output reaches the expected output. At 612, if the producedoutput is still greater than the expected output, the gaseous fuelsupply to that cylinder may be shutdown, as the gas admission valve maybe stuck open. In some embodiments, the cylinder may be operated withonly liquid fuel, or the cylinder may be totally shut down and no fuelsupplied to the cylinder. Further, a default action may be taken,indicating degradation of the gas admission valve (e.g., a diagnosticcode set, indicator lamp lit, etc.). Routine 600 then returns.

If the expected cylinder output is less than the expected output,routine 600 proceeds to 614 to increase the gaseous fuel supply untilthe output reaches the expected output. At 616, the amount of liquidfuel supplied to the cylinder may also be increased to maintain enginepower. For example, if the gas admission valve is stuck closed, anadequate supply of gaseous fuel may not be provided to maintainrequested engine power. To compensate, additional liquid fuel may besupplied. Further, a default action may be taken, indicating degradationof the gas admission valve (e.g., a diagnostic code set, indicator lamplit, etc.). Routine 600 then returns.

Thus, the systems and methods described herein provide for embodimentsfor reducing auto-ignition of end gases following compression ignitionof injected liquid fuel in a multi-fuel engine configured to operatewith both liquid and gaseous fuel. End gases may include the fuel-airmixture that has been introduced into the cylinder but not yet consumedin the flame-front reaction of the compression-ignition of the liquidfuel. In one example, a system comprises a first fuel system to deliverliquid fuel to at least one cylinder of an engine, a second fuel systemto deliver gaseous fuel to the at least one cylinder, and a controller.The controller is configured to control the supply of the gaseous fuelto the at least one cylinder, inject the liquid fuel to the at least onecylinder thereby to ignite the liquid fuel and the gaseous fuel in theat least one cylinder via compression-ignition, and adjust an amount ofgaseous fuel relative to an amount of liquid fuel based on a measuredparameter associated with auto-ignition of end gases subsequent to thecompression-ignition of the liquid fuel. The amount of gaseous fuel toliquid fuel (substitution rate) is not initially based on the presenceof auto-ignition, but on other parameters such as throttle notchposition, engine temperature, etc. The substitution rate may decrease(less gaseous fuel and more liquid fuel) only under auto-ignitionconditions when adjustments in liquid fuel injection timing does notreduce the auto-ignition as monitored via one or more sensors measuringcylinder vibrations.

The measured parameter may include engine vibrations detected by anaccelerometer, for example. The injection of the liquid fuel maycomprise a stratified injection of liquid fuel. In some examples, theliquid fuel is diesel fuel, and the first fuel system comprises a fueltank for holding the diesel fuel, a common fuel rail, and at least onefuel injector. The diesel fuel in the fuel tank may be supplied to thecommon fuel rail by at least one fuel pump, the common fuel rail may beconfigured to supply the diesel fuel to each fuel injector of the atleast one fuel injector, and each fuel injector of the at least one fuelinjector may be coupled to a respective cylinder of the at least onecylinder of the engine. The gaseous fuel may be natural gas, and thesecond fuel system may comprises a fuel tank to hold the natural gas,and at least one gas admission valve, each gas admission valve of the atleast one gas admission valve coupled to a respective cylinder of the atleast one cylinder of the engine. However, in some examples, rather thanincluding a plurality of gas admission valves, each coupled to arespective cylinder, the second fuel system may include a single gaseousfuel control valve to regulate an amount of gaseous fuel mixed withintake air upstream of the cylinders.

The controller may be configured to retard an injection timing of theinjection of liquid fuel in response to the measured parameter. Thecontroller may be configured to decrease the amount of gaseous fuelrelative to the amount of liquid fuel in response to the measuredparameter. In some examples, after decreasing the amount of gaseous fuelrelative to the amount of liquid fuel and responsive to the measuredparameter indicating that the auto-ignition of end gases has ceased, thecontroller may be configured to increase the amount of gaseous fuelrelative to the amount of liquid fuel. Further, after decreasing theamount of gaseous fuel relative to the amount of liquid fuel andresponsive to the measured parameter indicating that the auto-ignitionof end gases has not ceased, the controller may be configured deactivatethe supply of gaseous fuel to the at least one cylinder and increase theamount of liquid fuel to maintain engine power.

To decrease the amount of gaseous fuel relative to the amount of liquidfuel, one or more gas admission valves may be adjusted. For example, oneor more gas admission valves degree and/or duration of opening may bereduced to reduce the amount of gaseous fuel. In single-point fumigationsystems, to decrease the amount of gaseous fuel relative to the amountof liquid fuel, a gaseous fuel control valve may be adjusted to reducethe amount of gaseous fuel mixed with the intake air upstream of thecylinders (e.g., in the intake manifold or intake passage upstream ofthe intake manifold).

The at least one cylinder may comprise a first cylinder including afirst gas admission valve and a first liquid fuel injector, and a secondcylinder including a second gas admission valve and a second liquid fuelinjector. The controller may be configured to identify, based on themeasured parameter, if the auto-ignition is occurring in the firstcylinder, the second cylinder, or both the first and second cylinders.

If the auto-ignition is detected in both the first cylinder and thesecond cylinder, the controller is configured to reduce theauto-ignition by one or more of: retarding injection timing of liquidfuel injection from the first liquid fuel injector and the second liquidfuel injector; or increasing an amount of liquid fuel injected by thefirst fuel injector relative to an amount of gaseous fuel supplied bythe first gas admission valve and increasing an amount of liquid fuelinjected by the second fuel injector relative to an amount of gaseousfuel supplied by the second gas admission valve.

If the auto-ignition is detected in the first cylinder and not in thesecond cylinder, the controller is configured to reduce theauto-ignition in the first cylinder by one or more of: retardinginjection timing of liquid fuel injection from the first liquid fuelinjector while maintaining injection timing of liquid fuel injectionfrom the second liquid fuel injector; or increasing an amount of liquidfuel injected by the first fuel injector relative to an amount ofgaseous fuel supplied by the first gas admission valve while maintainingan amount of liquid fuel injected by the second fuel injector relativeto an amount of gaseous fuel supplied by the second gas admission valve.

The controller may be configured to identify a level of auto-ignition ineach of the first cylinder and second cylinder based on the measuredparameter, and if the level of auto-ignition in the first cylinder isabove a threshold level, the controller is configured to deactivate thefirst gas admission valve (e.g., maintain the gas admission valve in afully closed position) and increase the amount of liquid fuel injectedby the first fuel injector to maintain engine power.

In another example, a system comprises a first fuel system operable todeliver liquid fuel to a plurality of cylinders in an engine, the firstfuel system comprising a first fuel tank, a common fuel rail, and aplurality of fuel injectors, each fuel injector configured to injectliquid fuel to a respective cylinder of the plurality of cylinders; asecond fuel system operable to deliver gaseous fuel to the plurality ofcylinders, the second fuel system comprising a second fuel tank and aplurality of gas admission valves, each gas admission valve configuredto supply gaseous fuel to a respective cylinder of the plurality ofcylinders; and a control system. The control system is configured to,for each cylinder, combust a mixture of the gaseous fuel and air byinjecting liquid fuel to each cylinder of the plurality of cylinders,the gaseous fuel and liquid fuel provided at a first ratio; ifauto-ignition of end gases after primary ignition resulting from theinjection of the liquid fuel is detected in at least one cylinder,retard an injection timing of injection of the liquid fuel to the atleast one cylinder; and if the auto-ignition is still detected after theretarding of the injection timing, adjust at least one of an amount ofthe gaseous fuel or an amount of the liquid fuel provided to the atleast one cylinder to a second ratio, different than the first ratio.

The injection of liquid fuel may comprise stratified injection, and thecombustion of the mixture of gaseous fuel and air by the injection ofthe liquid fuel may comprise compression ignition of the liquid fuelwith the mixture of the gaseous fuel and air. The control system may beconfigured to, when adjusting said at least one of the amount of thegaseous fuel or the amount of the liquid fuel to the second ratio,increase the amount of liquid fuel relative to the amount of gaseousfuel.

In a further example, a method comprises supplying gaseous fuel to atleast one cylinder of an engine; compression igniting the gaseous fueland intake air by injecting liquid fuel to the at least one cylinder,the gaseous fuel and liquid fuel provided to the at least one cylinderat a substitution ratio; and in response to a measured parameterassociated with auto-ignition of end gases after the ignition resultingfrom the injection of the liquid fuel to the at least one cylinder,adjusting the substitution ratio.

The substitution ratio comprises a ratio of the gaseous fuel to theliquid fuel provided to the at least one cylinder, and adjusting thesubstitution ratio may comprise decreasing the substitution ratio.

The method may further comprise monitoring cylinder output resultingfrom combustion of the gaseous fuel at the substitution ratio; and ifthe cylinder output differs from an expected output by more than athreshold amount, indicating degradation of a gas admission valveconfigured to supply the gaseous fuel to the at least one cylinder. Themethod may further comprise, if the cylinder output is greater than theexpected output, decreasing one or more of an amount of the gaseous fuelor an amount of the liquid fuel supplied to the at least one cylinder,and if the cylinder output is still greater than the expected outputsubsequent to said decreasing of the one or more of the amount of thegaseous fuel or the amount of the liquid fuel, indicating the gasadmission valve is open by more than a desired amount and deactivatingthe supply of gaseous fuel. The method may further comprise, if thecylinder output is less than the expected output, increasing an amountof the liquid fuel supplied to the at least one cylinder until thecylinder output equals the expected output. The cylinder output maycomprise one or more of engine power or exhaust gas temperature.

In an embodiment, a system comprises a first fuel system to deliverliquid fuel to at least one cylinder of an engine, a second fuel systemto deliver gaseous fuel to the at least one cylinder, and a controller.The controller is configured to control the second fuel system forsupplying the gaseous fuel to the at least one cylinder, and to controlthe first fuel system for injection of the liquid fuel to the at leastone cylinder, for ignition of the liquid fuel, and thereby the gaseousfuel, by compression ignition. The controller is further configured tocontrol at least one of the second fuel system or the first fuel systemto adjust an amount of the gaseous fuel (supplied to the at least onecylinder) relative to an amount of the liquid fuel (injected into the atleast one cylinder) based on a measured parameter associated withauto-ignition of end gases subsequent to the compression-ignition of theliquid fuel.

In another embodiment of the system, the controller is configured tocontrol retarding of an injection timing of the injection of the liquidfuel in response to the measured parameter. The measured parametercomprises vibration of the engine.

In another embodiment of the system, the controller is configured tocontrol the at least one of the second fuel system or the first fuelsystem to decrease the amount of the gaseous fuel relative to the amountof the liquid fuel in response to the measured parameter. (For example,the second fuel system may be controlled to decrease the amount ofgaseous fuel while the first fuel system is controlled for the amount ofliquid fuel to remain static, increase, or even decrease but not to theextent that would prevent the ratio of gaseous fuel to liquid fuel fromdecreasing, or the first fuel system may be controlled to increase theamount of liquid fuel while the second fuel system is controlled for theamount of gaseous fuel to remain static, decrease, or even increase butnot to the extent that would prevent the ratio of gaseous fuel to liquidfuel from decreasing.)

In another embodiment of the system, the controller is configured to,after controlling decreasing the amount of gaseous fuel relative to theamount of liquid fuel and responsive to the measured parameterindicating that the auto-ignition of end gases has ceased, control atleast one of the second fuel system or the first fuel system to increasethe amount of gaseous fuel relative to the amount of liquid fuel. (Forexample, the second fuel system may be controlled to increase the amountof gaseous fuel while the first fuel system is controlled for the amountof liquid fuel to remain static, decrease, or even increase but not tothe extent that would prevent the ratio of gaseous fuel to liquid fuelfrom increasing, or the first fuel system may be controlled to decreasethe amount of liquid fuel while the second fuel system is controlled forthe amount of gaseous fuel to remain static, increase, or even decreasebut not to the extent that would prevent the ratio of gaseous fuel toliquid fuel from increasing.)

In another embodiment of the system, the controller is configured to,after decreasing the amount of gaseous fuel relative to the amount ofliquid fuel and responsive to the measured parameter indicating that theauto-ignition of end gases has not ceased, control the second fuelsystem to deactivate the supply of gaseous fuel to the at least onecylinder and control the first fuel system to increase the amount ofliquid fuel to maintain engine power.

In another embodiment of the system, the injection of the liquid fuelcomprises a stratified injection of the liquid fuel, e.g., thecontroller may be configured to control the first fuel system for theinjection of the liquid fuel into the at least one cylinder to be astratified injection.

In another embodiment of the system, the at least one cylinder comprisesa first cylinder including a first gas admission valve and a firstliquid fuel injector, and a second cylinder including a second gasadmission valve and a second liquid fuel injector. The controller isconfigured to identify, based on the measured parameter, if theauto-ignition is occurring in the first cylinder, the second cylinder,or both the first and second cylinders. According to another aspect, ifthe auto-ignition is detected in both the first cylinder and the secondcylinder, the controller is configured to reduce the auto-ignition byone or more of: controlling the first fuel system to retard injectiontiming of the liquid fuel injection from the first liquid fuel injectorand the second liquid fuel injector; or controlling at least one of thefirst fuel system or the second fuel system to increase an amount ofliquid fuel injected by the first fuel injector relative to an amount ofgaseous fuel supplied by the first gas admission valve and increase anamount of liquid fuel injected by the second fuel injector relative toan amount of gaseous fuel supplied by the second gas admission valve.According to another aspect, additionally or alternatively, if theauto-ignition is detected in the first cylinder and not in the secondcylinder, the controller is configured to reduce the auto-ignition inthe first cylinder by one or more of: controlling the first fuel systemto retard injection timing of liquid fuel injection from the firstliquid fuel injector while maintaining injection timing of liquid fuelinjection from the second liquid fuel injector; or control at least oneof the first fuel system or the second fuel system to increase an amountof liquid fuel injected by the first fuel injector relative to an amountof gaseous fuel supplied by the first gas admission valve whilemaintaining an amount of liquid fuel injected by the second fuelinjector relative to an amount of gaseous fuel supplied by the secondgas admission valve. The controller may be further configured toidentify a level of auto-ignition in each of the first cylinder andsecond cylinder based on the measured parameter, and if the level ofauto-ignition in the first cylinder is above a threshold level, thecontroller is configured to control the second fuel system to deactivatethe first gas admission valve and to control the first fuel system toincrease the amount of liquid fuel injected by the first fuel injectorto maintain engine power.

In another embodiment of the system, the liquid fuel is diesel fuel. Thefirst fuel system comprises a fuel tank for holding the diesel fuel, acommon fuel rail, at least one fuel injector, and at least one fuelpump. The diesel fuel in the fuel tank is supplied to the common fuelrail by the at least one fuel pump. The common fuel rail is configuredto supply the diesel fuel to each fuel injector of the at least one fuelinjector. Each fuel injector of the at least one fuel injector iscoupled to a respective cylinder of the at least one cylinder of theengine.

In another embodiment of the system, the gaseous fuel is natural gas.The second fuel system comprises a fuel tank and at least one gasadmission valve. Each gas admission valve of the at least one gasadmission valve is coupled to a respective cylinder of the at least onecylinder of the engine.

In another embodiment of the system, the liquid fuel is diesel fuel andthe gaseous fuel is natural gas. The first fuel system comprises a fueltank for holding the diesel fuel, a common fuel rail, at least one fuelinjector, and at least one fuel pump. The diesel fuel in the fuel tankis supplied to the common fuel rail by the at least one fuel pump. Thecommon fuel rail is configured to supply the diesel fuel to each fuelinjector of the at least one fuel injector. Each fuel injector of the atleast one fuel injector is coupled to a respective cylinder of the atleast one cylinder of the engine. The second fuel system comprises afuel tank (for storage of the natural gas) and at least one gasadmission valve. Each gas admission valve of the at least one gasadmission valve is coupled to a respective cylinder of the at least onecylinder of the engine.

In a further embodiment, a system comprises a first fuel system operableto deliver liquid fuel to a plurality of cylinders in an engine, thefirst fuel system comprising a first fuel tank, a common fuel rail, anda plurality of fuel injectors, each fuel injector configured to injectliquid fuel to a respective cylinder of the plurality of cylinders; asecond fuel system operable to deliver gaseous fuel to the plurality ofcylinders, the second fuel system comprising a second fuel tank andgaseous fuel control valve configured to supply gaseous fuel to theplurality of cylinders; and a control system. The control system isconfigured to, for each cylinder, combust a mixture of the gaseous fueland air by injecting liquid fuel to each cylinder of the plurality ofcylinders, the gaseous fuel and liquid fuel provided at a first ratio;if auto-ignition of end gases after primary ignition resulting from theinjection of the liquid fuel is detected in at least one cylinder,retard an injection timing of injection of the liquid fuel to the atleast one cylinder; and if the auto-ignition is still detected after theretarding of the injection timing, adjust at least one of an amount ofthe gaseous fuel or an amount of the liquid fuel provided to the atleast one cylinder to a second ratio, different than the first ratio.The amount of gaseous fuel may be adjusted by controlling the positionand/or opening duration of the gaseous fuel control valve.

As explained above, the terms “high pressure” and “low pressure” arerelative, meaning that “high” pressure is a pressure higher than a “low”pressure. Conversely, a “low” pressure is a pressure lower than a “high”pressure.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. A system, comprising: at least one fuelsystem configured to supply liquid fuel and gaseous fuel to at least onecylinder of an engine; and a controller storing non-transitoryinstructions executable to: initiate compression ignition of the gaseousfuel and intake air by controlling injection of the liquid fuel to theat least one cylinder, the gaseous fuel and liquid fuel provided to theat least one cylinder at a substitution ratio; wherein the controller isfurther configured, in response to a measured parameter associated withauto-ignition of end gases after the compression ignition resulting fromthe injection of the liquid fuel to the at least one cylinder, to adjustthe substitution ratio; and retard an injection timing of the injectionof the liquid fuel in response to the measured parameter.
 2. The systemof claim 1, wherein the substitution ratio comprises a ratio of thegaseous fuel to the liquid fuel provided to the at least one cylinder,and wherein the controller is configured to adjust the substitutionratio by decreasing the substitution ratio which includes decreasing anamount of the gaseous fuel relative to an amount of the liquid fuel, andwherein the measured parameters comprise vibration of the engine.
 3. Thesystem of claim 1, wherein: the controller is further configured tomonitor cylinder output resulting from combustion of the gaseous fuel atthe substitution ratio; and the controller is further configured,responsive to the cylinder output differing from an expected output bymore than a threshold amount, to indicate degradation of a gas admissionvalve configured to supply the gaseous fuel to the at least onecylinder.
 4. The system of claim 3, wherein the controller is furtherconfigured, responsive to the cylinder output being greater than theexpected output, to decrease one or more of an amount of the gaseousfuel or an amount of the liquid fuel supplied to the at least onecylinder, and responsive to the cylinder output being still greater thanthe expected output subsequent to the one or more of the amount of thegaseous fuel or the amount of the liquid fuel being decreased, toindicate the gas admission valve is open by more than a desired amountand deactivate the supply of gaseous fuel.
 5. The system of claim 3,wherein the controller is further configured, responsive to the cylinderoutput being less than the expected output, to increase an amount of theliquid fuel supplied to the at least one cylinder until the cylinderoutput equals the expected output, and wherein the cylinder outputcomprises one or more of engine power or exhaust gas temperature, andwherein an injection of the liquid fuel comprises a stratified injectionof liquid fuel.
 6. A vehicle comprising the system of claim
 1. 7. Asystem, comprising: a first fuel system operable to deliver liquid fuelto a plurality of cylinders in an engine, the first fuel systemcomprising a first fuel tank, a common fuel rail, and a plurality offuel injectors, each fuel injector configured to inject liquid fuel to arespective cylinder of the plurality of cylinders; a second fuel systemoperable to deliver gaseous fuel to the plurality of cylinders, thesecond fuel system comprising a second fuel tank and a plurality of gasadmission valves, each gas admission valve configured to supply gaseousfuel to a respective cylinder of the plurality of cylinders; and acontroller configured to, for each cylinder of one or more of theplurality of cylinders: initiate compression ignition of the gaseousfuel and intake air by controlling injection of the liquid fuel to therespective cylinder, the gaseous fuel and liquid fuel provided to therespective cylinder at a first substitution ratio; in response to ameasured parameter associated with auto-ignition of end gases after theignition resulting from the injection of the liquid fuel to therespective cylinder, retarding an injection timing of the liquid fuel tothe respective cylinder; and in response to the measured parameterassociated with auto-ignition still being present after retarding theinjection timing, to adjust the first substitution ratio by adjusting atleast one of an amount of the gaseous fuel or an amount of the liquidfuel to a second substitution ratio different than the firstsubstitution ratio.
 8. The system of claim 7, wherein adjusting at leastone of the amount of the gaseous fuel or an amount of the liquid fuel tothe second ratio comprises increasing the amount of liquid fuel relativeto the amount of gaseous fuel.
 9. The system of claim 7, wherein: thecontroller is further configured to monitor a cylinder output resultingfrom combustion of the gaseous fuel at the first or second substitutionratios; and the controller is further configured, responsive to thecylinder output differing from an expected output by more than athreshold amount, to indicate degradation of a gas admission valveconfigured to supply the gaseous fuel to the respective cylinder. 10.The system of claim 9, wherein the controller is further configured,responsive to the cylinder output being greater than the expectedoutput, to decrease one or more of an amount of the gaseous fuel or anamount of the liquid fuel supplied to the respective cylinder, andresponsive to the cylinder output being still greater than the expectedoutput subsequent to the one or more of the amount of the gaseous fuelor the amount of the liquid fuel being decreased, to indicate the gasadmission valve is open by more than a desired amount and deactivate thesupply of gaseous fuel.
 11. The system of claim 9, wherein thecontroller is further configured, responsive to the cylinder outputbeing less than the expected output, to increase an amount of the liquidfuel supplied to the respective cylinder until the cylinder outputequals the expected output, and wherein the cylinder output comprisesone or more of an engine power or an exhaust gas temperature.
 12. Thesystem of claim 7, wherein the first fuel tank is on a first vehicle,and the second fuel tank is on a second, fuel tender vehicle that iscoupled to the first vehicle.
 13. The system of claim 12, wherein thecontroller and the engine are on the first vehicle.
 14. A vehiclecomprising the system of claim 7.